SAPROPHYTES 407 



which these bacteria combine within the nodules later be- 

 comes available to the host plant and is built up into its proto- 

 plasm. When the host plant dies and disintegrates, the 

 nitrogen thus fixed by the nodule bacteria becomes a part of 

 the soil. It has long been known by practical farmers that 

 plants belonging to this clover family tend to increase the 

 productivity of the soil on which they are grown. We now 

 know, however, that it is not the higher plant, but the nodule 

 bacteria that are directly responsible for this increased fertility. 

 464. Effect of Cropping on Soil Nitrogen. Thus, in these 

 last two groups of bacteria, we have agencies which trans- 

 form some of the atmospheric nitrogen into forms in which it 

 becomes a part of the soil and available to the higher plants. 

 In the first of these three groups, we have agencies which 

 ultimately return the nitrogen to the soil which the higher 

 plants take from it. In a state of wild nature, in which all 

 the plants that grow on the land die where they stand, and 

 ultimately disintegrate and return their nitrogen to the soil, 

 we naturally have a gradual increase in the soil nitrogen. 

 This generally means* a gradual increase in the fertility of the 

 soil and in the luxuriance in the plant growth that it supports. 

 Under modern agriculture, however, through which the larger 

 portion of the plants grown on the land is often removed, there 

 is danger that the soil nitrogen may be reduced. If the 

 nitrogen removed from the land in the crops together with that 

 which is carried away in the drainage, is greater than that 

 added by bacteria and by manures and other fertilizers, there 

 must be a decrease in the amount of nitrogen remaining in the 

 soil. If this is kept up for many years, the soil is gradually 

 impoverished, and it may in the end become too unproductive 

 to pay for the work of tillage. This thing has actually oc- 

 curred in many originally fertile soils of the world. All of 

 the older states of this country contain many abandoned 

 farms which have become so unproductive through the ex- 

 haustion of the nitrogen, and possibly of other elements of 

 fertility, that their owners have ceased to till them. 



